In a hydraulic power unit which actuates a double acting cylinder using a two-way pump and thereby vertically moves a ram, the pressure of the double acting cylinder is in a negative pressure state when the cylinder starts moving the ram. Due to this, dead time occurs to the ram before actuating the two-way pump, supplying hydraulic oil to the double acting cylinder and moving the ram. Because of this ram dead time, a shock is generated on the ram. To avoid this shock phenomenon, the ram is warmed up.
According to a conventional hydraulic power unit ram moving method, therefore, if the movement of the ram is started according to a ram moving pattern, a speed distribution instruction to the ram is clamped at a predetermined speed for constant timer time before the moving speed reaches the highest speed, thereby warming up the ram.
However, the hydraulic power unit which employs such a two-way pump has a disadvantage in that ram dead time is not constant but varies depending on the operating state of the ram just before the dead time. That is, according to the ram moving method by utilizing constant timer time, a shock is generated because of too short timer time or, conversely, that time is adversely influenced by too long timer time.
Meanwhile, an axial plunger pump which is used as the two-way pump is constituted so that a cylinder block, which includes a plurality of plungers (pistons) provided therein to be able to be reciprocated, is rotatably disposed in a casing and that the rotation of this cylinder block reciprocates the plungers sequentially. The axial plunger pump is also constituted to suck hydraulic oil when each plunger gradually moves from a top dead center position (which is a position at which the plunger is in an engaged state in which the plunger is fitted into the cylinder block most deeply) to a bottom dead center position (which is a position at which the plunger is in an engaged state in which the plunger is fitted into the cylinder block most narrowly) and, conversely, to discharge the hydraulic oil when the plunger moves from the bottom dead center position to the top dead center position.
A valve plate which is fixedly provided in the casing to correspond to the cylinder block, is equipped with a circular arc-shaped suction port and a circular arc-shaped discharge port to correspond to the moving position in a plunger suction step and that in a plunger discharge step, respectively.
The suction port and the discharge port are provided to be slightly away from each other. A region between the suction port and the discharge port covers both the position which corresponds to the top dead center and the position which corresponds to the bottom dead center of the plunger. At the top dead center and bottom dead center of the plunger, a plunger insertion hole through which the plunger is inserted into the cylinder block is shielded, thereby causing a confinement phenomenon.
If the plunger is located at the bottom dead center position, the plunger is about to move to the discharge step and the pressure of the confined hydraulic oil is low. Due to this, the confinement phenomenon does not cause a significant problem. However, if the plunger is located at the top dead center position, the pressure of the confined hydraulic oil is high. If the plunger moves from the top dead center to a suction port side, pressure has great change, thereby causing vibration and noise.
To prevent the confinement phenomenon at the top dead center position of the plunger, there is proposed forming a notch, which communicates this confinement position with the discharge port, in the valve plate.
In the axial plunger pump, the rotation of the cylinder block is not limited to forward rotation but the cylinder block is sometimes rotated in a counter direction. In this case, the suction port and the discharge port change places with each other. Therefore, to prevent the confinement phenomenon, the above-stated notch can be provided in each of the suction port and the discharge port.
Nevertheless, the valve plate and the cylinder block are not located to be proximate to each other. Although depending on an operation state, a small distance of about 0.01 mm exists between the valve plate and the cylinder block. For that reason, the notch may cause the leakage of the hydraulic oil. If the notch is too large, pumping performance deteriorates. If the notch is too small, it does not sufficiently contribute to the prevention of vibration and that of noise. Thus, the axial plunger pump has a disadvantage in that it is difficult to form an appropriate notch.
The objects of this invention have been derived while paying attention to the above-stated technical disadvantages.
It is, therefore, the first object of the present invention to provide a ram moving method in a hydraulic power unit and a ram moving controller in a hydraulic power unit capable of moving a ram in shortest time without causing a shock even if the dead time of the ram changes.
It is the second object of the present invention to provide an axial plunger pump confinement prevention method and an axial plunger pump capable of effectively preventing vibration and noise.